Vapor phase crystallization



Feb. 11, 196% H. J. GoULD 3,121,062

' VAPOR DHAsLi CRYSTALLIZATION File@ June 22, 1961 v Y y J., l

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INVENTQR 'United 1 3,121,062 VAPOR PHASE CRYSTALLZATIQN Herbert J. Gould, 4818 Wilimonte Ave., Temple City, Calif.

Filed .lune 22, v1961, Ser. No. H8337 5 Claims. (Cl. 252-62.3)

This invention has to do with the crystallization from the vapor phase of crystals at least one component of which is a metal that is solid at normal temperature.

The invention has to do, more particularly, with irnproved methods and apparatus for supplying to a furnace chamber the vapor of a metallic component that is solid at normal temperature.- v

The invention is particularly useful in, but is not limited to, the production of semiconductive crystals, such, for example, as cadmium and zinc sulphide, selenide and telluride and mixtures of such components. It is well known that crystals ofthat type can be produced in good purity by vapor phasccrysiallization. v

In producing such illustrative crystals the non-metallic component is ordinarily supplied to the furnace as a con-` tinuous stream of gas, typically as hydrogen sulphide, selenide or telluride. The rate of supply' of such a gaseous component isA accurately and conveniently controllable by knownmethods. l

The metallic component is ordinarily supplied lby inserting in the furnacein the gas stream a -boat containing the vmetal in solid orliquid form. The metal thenevap orates into the gas stream at a rate that is roughly controllable by variation of such factors as the temperature, the rate of gas ilozv and the arca of theexposed metal surface.

Such control, however, is less accurate than is often desirable, and is unsatisfactory for many other reasons. Even rough control of the vaporzing temperature usually requires a special furnace zone for that purpose. The rate of gas flow that is most suitable for metal vaporiza-- tion may be undesirable for other reasons. And the area way as the initial charge is exhausted. Moreover, the rate of evaporation from a given surface arca is very sensitive to contamination of the surface, which typically increases as vthe metal charge is consumed.

The present invention avoids all of those ditlcultics in a remarkably economical and convenient manner. In accordance with one aspect of the invention, the metallic component is supplied in the form of a fine wire, and is fed to the furnace at 'a definite velocity. The metal vaporizes continuously as it reaches the point of vaporizing temperature, The rate of metal-vapor supply to the furnace chamber is then conveniently and accurately controllable by mechanical variation of the feed velocity. The described method has the further great advantage that under equilibrium conditions of operation the rate of vapor supply is essentially or completely independent of virtually all other variable factors. Hence those factors may be adjusted arbitrarily as 'required to meet other conditions. y

In accordance with a further aspect of the invention, the metal wire is fed to the furnace chamber through a capillary passage formed of suitable inert material, such as quartz, for example. The passage is so arranged that the advancing metal reaches vaporizing temperature at a point spaced from the exit mouth of the passage. Vaporization then occurs within the capillary' passage, and the metal leaves the passage mouth as a continuous and uniform stream of vapor. A further advantage of that structure is that, by suitable form and placement of the passage, the metal vapor can be delivered accurately to any desired point of the furnace chamber.

A further aspect of the invention provides means for surface cleaning of the metal Wire immediately prior to 'of metal surface usually decreases in anvuncontrollable ZlZ Patented Feb. 1l, 1964 melting and vaporization. That may be accomplished by passing the wire through a reducing chamber which is continuously' Washed by a reducing gas, such as hydrogen, for example. The reducing chamber is preferably maintained at anvelcvated temperature, which may be only 'slightly less than the melting point of the metal.

A full understanding of the invention, and of its further objects and advantages, will be had from the following description of certain illustrative manners in which it may be carried out, of which description the accompanying drawings form a part. The particulars of that description are intended only as illustration, and not as a limitation upon the scope of the invention, which is dened in the appended claims.

In the drawings:

FIG. 1 is a schematic drawing of illustrative apparatus in accordance with the invention, including a schematic axial section of a typical furnace;

FIG. 2 is a fragmentary detail of FIG. 1 at enlarged scale; y

FIG. 3 is a fragmentary section representing a modifcation.

In the drawings an illustrative furnace tube is represented at 10, typically constructed of fused quartz. Electrical heating coils are represented at v12 and 14, with respective power sources 13 and l5. An apertured baille 16 partially divides the furnace chamber into two Zones 17 and 18, whch'may be maintained at different temperaturcs. Temperature control apparatus is typically incorporated in the power sources, permitting the temperatures of the respective furnace zones to be separately controlled, as by adjustment of calibrated dials indicated at 13a and 15a. Both ends of furnace tube 10 are typically closed by apertured plugs, shown at 20 and 22.

F or growing a typical crystal derived from one gaseous component and one metallic component, the gaseous com ponent is introduced into zone 17 of the furnace tube via an entrance tube 24, which is typically of quartz and mounted in a bore in plug 20. The desired gas such, for example, as hydrogen sulphide, hydrogen selenide or hydrogen telluride may be supplied to tube 24 from a suitable tank or generator 25 via a flow control device 26, which may be of conventional type. Means of known-type may also be provided for Washing furnace tube 10 with any desired inert or reagent gas prior to each crystallizing operation. An outlet tube 23 is provided in end plug 22, leading typically from zone 18 to the atmosphere, or to suitable disposal apparatus. If the furnace is to be operated at reduced pressure, for example, outlet tube 23 may be connected to a vacuum pump.

In'accordance with one aspect of the present invention, a capillary tube 30, preferably of fused quartz, is mounted, as in a bore in plug 20, with its inlet end 31 outside of the furnace and its outlet end 32 within the furnace at any desired point. As iilustratively shown, tube outlet 32 is in furnace zone 17 in the general vicinity of baffle 16. However, the tube outlet may be placed at substantially any position and with any direction of delivery that may be desired.

A metal wire is represented at 40, consisting essentially of the metal component required as vapor for reaction and crystallization with the gas from tube 24. As an illustration, the wire may be formed primarily of cadmium or zinc. Wire 40 is taken typically from a supply spool 42 and is fed between two feed rolls 44 and 45 into inlet end 31 of capillary 30. The feed rolls are preferably made of a slightly yieldino and chemically inert material such, for example, as polymerized tetrauoroethylene, which is available commercially under the trade name Teflon. The rolls are driven from a suitable motor and speed reduction gear, indicated schematically at 48, vvia adjacent the tube.

v tion. v

a driving connection 49. Connection 49 preferably includes a continuously variable speed control, represented schematically at 50, with speed adjusting dial 52'). from the known wire diameter and density, a suitable feed welocity can then be obtained conveniently, to supply imetal to the furnace. a'tV any desired rate. In practical crystallizin'g processes for making semiconductors of the l.type described, suitable feed rates are typically obtainable with wire sizes inthe range from 30 gauge to 20 gauge and with feed' velocities from about l mm.. to about 10 imm. per minute..

With the typical arrangement o f FIG. l, the metal' vapor issues from tube 30 at 32 into furnace zone 17, which contains essentially an atmosphere of gas from tube 24. `That; gasV is maintained above the vaporizing temgperature of the metal, so that vaporization occurs typically well within tube- 30. The gas and. vapor mixture passes :throughy baflle 16 into furnace lzone f8, which is maintained at a suitablecrystallizing temperature. A suitable ibase for crystal formation and growth may be provided in chamber 18 and is indicated schematically at 60 with growing crystals represented at 62. The residual gas and vapor leave via tube 23.

In accordance withy a further aspect of the invention,

the advancingl wire 40 is caused to pass through a rela# portion 35 just beyond the sleeve, which is exposed to substantially the full heat of furnace chamber 17. That gradient. is preferably much steeper than the temperature gradient existing in the furnace chamber itself directly The portion of tube at the high temperature gradient is shownat enlarged scale in FIG. 2. As wire 4G advances to the right vas seen in the gures, it reaches melting ternperature at a point indicated typically at Tm, forming a drop of molten metal 64. As fresh liquid is formed it is prevented from flowing back to the left by surface causes-a small shift of equilibrium position of surface 68, leading to rapid stabili/.ation ot' vaporization at the newv equilibrium rate. Thus the vapor feed rute is controllable accurately and essentially independently` of other operating conditions.

With previous methods of supplying metal vapor, if the metal to be vaporized includes a definite proportion of impurity, whether present accidentally or intentionally, the additiveusually melts and evaporates at a different rate from the metal. Such fractionation causes progressive changes in concentration of the impurity, both in the vapor produced and in the remaining melt. The crop of crystals is therefore non-uniform, the initial growth, for example, containing more than average concentration of lower boiling impurities and less than average of higher boiling impurities. v

With the vapor supply'method of the present invention, that difficulty is virtually eliminated. If an impurity initially evaporates from surface 68 at a lower rate, for example, than is. proportional to its concentration, the

result is to increase the impurity concentration in the liquid closelyadj'acent surface 68. The rate of vaporizav tion is thereby progressively increased, until a proporlwhich is cooled by conduction along the sleeve, and the tension at the faces 65 and by the gradually advancing ,i

wire. Theliquid therefore moves to the right, advancing the front face 68 of the drop progressively along the tube to higher temperatures. At some point, denoted Tv in FIG. 2, a vaporizing temperature is reached at which the rate of metal evaporation from surface 68 just equals the rate at which metal arrives at the surface from the left as liquid. Under such equilibrium vaporizing surface 68 remains xed, and the rate of vaporization corresponds accurately to the rate of feed of solid wire 40.

If some condition, such as furnace temperature, is then changed slightly, vaporizing surface 63 shifts correspondingly to a new equilibrium position. That shift involves a temporary change in rate of vaporization. However, the magnitude of such-change is inherently small if the temperature change is gradual; and is further limited by the small sectional area of the capillary bore. Further, the steeper the temperature gradient near surface 68, the smaller the axial movement of that surface to its new equilibrium position. Byl use of a suitably small capillary, and correspondingly tine wire, and by providing suicientlysteep temperature gradient in the neighborhood of Tv, the rate of vaporization may be made essentially independent of other factors of the furnace opera- That operation may therefore be adjusted with great freedom to obtain optimum conditions without disl turbing the rate of supply'of metal vapor.

Also, variation of the 'feed rate of wire 40 typically tional rate is attained'. Once equilibrium is established, the vapor leaving tube 30 contains the same proportion of impurity. as. the wire entering it. An important point is that equilibrium is established rapidly, due primarily to the small area of surface 68 and to the small volume of liquid adjacent that surface. A steep temperature gradi-V ent is also helpful in accelerating this type of equilibrium.

Because of the action just described, the method of the invention is well adapted for growing crystals of uni.v

form composition. On the one hand,v with metal of high initial purity, the resulting crystals are correspondingly pure and are not degraded, as is usual with previous methods, by concentration of the impurities in the initial or in the final' fractions of the growth.

On the other hand, the present method for the first time permits crystals to be grown from the vapor phase con. taining an accurately controllable and closely uniform concentration of desired impurities.

As an example, it is well known that the photoconductive properties of such semiconductive crystals as cadmium sulphide depend very sensitively upon the concentration of certain impurities, commonly referred to as doping agents. Many such doping agents are well known, including in particular the elements boron, aluminum, gallium, indium, phosphorus, arsenic, antimony and copper, aswell as many others. Such doping agents can be added to cadmium metal in the desired concentrations, which are typically extremely small; but it has not previouslyv been possible to produce metal vapor containing an equal, or even a uniform, concentration. With the present method, the accurately doped and thoroughly mixed cadmium, or other vaporizable metal, is drawn or otherwise formed by conventional procedures into a wire of suitable diameter. When the wire is fed to the furnace as already described, any tendency for the dopingagent to segregate at the melting or vaporizing face reaches equilibrium rapidly. Thereafter the vapor leaving tube 30 contains a concentration of doping agent that remains essentially uniform and is accurately equal to the selected concentration that is present in wire 40.

FIG. 3 is illustrative of the wide range of modifications that may be made within the scope of the present invention. Furnace tube 10a contains only a single furnace zone, the temperature of which is controlled in conventional manner, as by regulation of power supplied to heating coil 12a. The metal wire 40 is fed into the left end of tube 39a, typically in the illustrative manner already described in connection with FIG. l.

Metal purifying apparatus in accordance with the invention is indicated generally at 70. The bore of tube 30a is locally enlarged, preferably' at a point just within the furnace, to form a chamber 71 through which wire 40 surface traces of cadmium oxide.

ployed in each instance is selected, with'due regard for the known chemical characteristics of the wire and the crystal by vapor phase crystallization, said method comprising providing a capillary passage communicating with the chamber, feeding a wire consisting essentially ofthe impurities that need to be eliminated.- For example, a'-

wire Vof cadmium typically becomes contaminated with effectively removed by providing inchamber 70 an atmosphere containing hydrogen at an elevated temperature below themelting poirit of cadmium. Such temperature is usually readily obtainable by placing chamber 90 in a relatively cool part of the furnace, for example near end wall a as illustrated. Auxiliary heating or cooling means of conventional .type may be provided. The hydrogen gas is effectively inert with respect to cadmium metal, but reduces the cadmium oxide'impurity to cadmium metal and water vapor, which is carried off through outlet tube 74 with the gas stream.

FIG. 3l further illustrates provision of means for locally heating tube a to produce a sharp axial temperature gradient. A source of controlled high frequency alternating current power is indicated schematically at 80. Power leads 81 and 82 are mounted in a suitable insulating bushing 83 in furnace plug 20a for supplying powerfrom source 8 0 to the induction heating coil 84. Coil 84 surrounds tube 30a at a point which is normally cooler than vaporizing temperature of the metal of wire 40. It -is ordinarily advantageous that such induction heatingtends to supply heat directly to the metallic wire 40, providing accurate Control of the resulting temperature gradient in the wire. That gradient is indicated schematically in FIG. 3 by the temperature designations Tm and Tv, corresponding generally to the similar designations in FIG. 2. Alternatively, a metal sleeve may be mounted on tube 30a within induction coil 84. Heat is-then supplied primarily to the sleeve and by conduction therefrom through .tube 30a to the wire.

Many other known techniques for supplying and controlling the ow of heat may be employed for producing Such traces can be metal into the passage toward the chamber at a controlled velocity, and maintaining the chamber at such temperature that the wire is vaporized within the passage.

2. The method of supplying vapor of a metal at a controlled rate to a furnace chamber for production of a crystal by vapor phase crystallization, said method comprising providing a capillary passage'that opens into the chamber, maintaining in the passage at a point spaced from said opening a longitudinal temperature gradient that is steeper than the gradient in the chamber adjacent the. passage and that embraces the melting temperature and the vaporizing temperature of the metal, and feeding a wire consisting essentially of the metal into the passage toward the chamber at a controlled velocity.

3. The method of supplying vapor of a metal at a controlled rate to a furnace chamber for production of a crystal by vaporphase crystallization, said method comprising providing a passage communicating at one end .with the chamber, feeding a wire consisting essentially of the metal into the passage toward the chamber at a controlled velocity, maintaining the passage at such temperature that the wire is vaporized therein adjacent said passage end, and contacting the wire in the passage prior to said vaporization with a gas that is substantially inert withV respect to the metal and that chemically reacts with impurities carried by the wire.

4. The method of producing a semiconductive crystal comprising a metallic component and containing a substantially uniform Vrelative concentration of a doping agent, said method comprising providing a solid wire which consists essentially of the metallic component and the doping agent in said relative concentration, feeding the the wire at a controlled velocity into a capillary tube,

'maintaining a temperature gradient along the tube to the desired temperature gradient within the scope of the present invention.

FIG. 3 further illustrates means for delivering the stream of metal vapor from the outlet 32a of tube 30a directly into the stream of fresh gas from gas inlet tube 24a. In the illustrative arrangement shown, tube 24a opens into a horn structure 90 which coaxially surrounds the outlet portion of tube 30a. The fresh gas and metal vapor' areV then combined in the horn prior to their exposure to the general atmosphere of the furnace. Suitable surfaces of known type for initiating crystallization may be provided in any desired relation to horn 90. Such means are represented schematically at 92, with growing` crystals indicated at 94.

1. The method of supplying vapor of a metal at acontrolled rate to a-furnace chamber for production of a form said semiconductive crystal.

5. The method of producing a semiconductive crystal containing substantially uniformly distributed therein a minor proportion of a doping agent, said method comprising providing a solid wire composed primarily of metal selected from the group consisting of cadmium and zinc and containing said doping agent in a concentration corresponding to said proportion, feeding the wire at controlled velocity into a capillary tube, maintaining a temperature gradient along the tube to vaporize the wire, combining the resulting vapor in substantially constant ratio with gas selected from the group consisting of hydrogen sulphide, hydrogen selenide and hydrogen telluride, and crystallizng said semiconductive crystal from the resulting vapor phase.

References Cited in the file of this patent u UNITED STATES PATENTS 2,810,052 Bube et al. Oct. 15, 1957 2,969,448 Alexander Jan. 24, 1961 FOREIGN PATENTS 878,884 Germany June 8, 1953 

5. THE METHOD OF PRODUCING A SEMICONDUCTIVE CRYSTAL CONTAINING SUBSTANTIALLY UNIFORMLY DISTRIBUTED THEREIN A MINOR PORPORTION OF A DOPING AGENT, SAID METHOD COMPRISING PROVIDING A SOLID WIRE COMPOSED PRIMARILY OF METAL SELECTED FROM THE GROUP CONSISTING OF CADMIUM AND ZINC AND CONTAINING SAID DOPING AGENT IN A CONCENTRATION CORRESPONDING TO SAID PROPORTION, FEEDING THE WIRE AT CONTROLLED VELOCITY INTO A CAPILLARY TUBE, MAINTAINING A TEMPERATURE GRADIENT ALONG THE TUBE TO VAPORIZE THE WIRE, COMBINING THE RESULTING VAPOR IN SUBSTANTIALLY CONSTANT RATIO WITH GAS SELECTED FROM THE GROUP CONSISTING OF HYDROGEN SULPHIDE, HYDROGEN SELENDIE AND HYDROGEN TELLURIDE, AND CRYSTALLIZING SAID SEMICONDUCTIVE CRYSTAL FROM THE RESULTING VAPOR PHASE. 